When an aircraft makes contact with the ground it is travelling at high speed and it is necessary to bring it to a halt as quickly as possible in as short a distance as possible.
An aircraft generally lands by using running means, eg. wheels forming part of its landing gear, to make contact with the runway. The best known means for braking an aircraft once it has made contact with the ground consists in using friction brakes, eg. disk brakes, for absorbing energy. Such disk brakes are generally constituted by rotors and stators which are pressed against one another other by piston means driven by hydraulic fluid under pressure.
It should be mentioned that aircraft brakes present considerable design difficulties since they have to satisfy extremely severe criteria concerning even higher levels of safety and reliability than are required for other vehicles. To meet these criteria, a whole set of relatively complex devices are implemented to obtain aircraft deceleration in complete safety while preventing, for example, the wheels from locking, from sliding on puddles, etc.
The most usual braking method implemented on such vehicles consists in measuring the speed of the aircraft relative to the ground while it is running thereon, and in controlling the wheel braking by a servo system acting on the said measured aircraft speed.
Very briefly and without going into detail, this is achieved by ensuring that the equivalent linear speed of the braking wheels (generally the wheels on the main landing gear) should not drop to below some given quantity less than the aircraft ground speed. Experience has shown that this quantity should be about 15%. This kind of method provides adequate safety levels and optimizes braking forces.
In implementing this method, one of the problems that is encountered is the problem of measuring the true speed of the aircraft relative to the runway as accurately as possible. This is done by means of a sensor mounted on one of the aircraft wheels, generally the nose wheel. The nose wheel is in contact with the ground and is not used for braking, so in theory, its equivalent linear speed should be identical to the aircraft speed relative to the ground.
However, experience shows that using the nose wheel to measure ground speed is not always reliable. The nose wheel may slip or skid if the ground is wet or icy, or the nose wheel may itself be damaged for various reasons. This can result in the ground speed measurement being incorrect in some cases, and thus not always suitable. Unsuitable ground speed measurements will thus lead to the main landing gear brakes being servocontrolled to a wrong value, which can lead to disasters such as tire bursts or no braking at all.
Further, given current requirements of economy, speed, space saving, and shortness of runways, there is continuing demand for aircraft that can be halted ever more quickly in ever shorter distances.
To do this, it would be an advantage if all the wheels of the aircraft that are in contact with the ground could be used for braking, including the nose wheel. In which case, with all the wheels being braked, it is no longer easy to measure the true ground speed of the aircraft.
Preferred implementations of the invention thus provide a method of braking control in which the above-mentioned drawback is obviated, while still enabling all of the wheels of a vehicle, eg. an aircraft, which are in contact with the ground to be used for braking.